The radiolysis of hexene-1, hexene-2, and hexene-3 at room temperature was studied with particular emphasis on the mechanisms leading to dimer formation. The structures of the hexene dimers were investigated by means of infrared and n.m.r. spectroscopy. The dimers from hexene-1, -2, and -3 had average unsaturations of 1.0, 1.2, and 1.2 double bonds/molecule, respectively, and were formed with G values of 4.6, 4.6, and 4.1 respectively. The dimerization of hexene-1 could be interpreted as proceeding, in part, through an ionic mechanism leading exclusively to monoolefinic structures, and in part through another ionic mechanism leading to a mixture of saturated, monoolefinic, and diolefinic structures. Hexene-2 and hexene-3 were found not to dimerize by the first of these ionic mechanisms, but largely by the second mechanism, along with some contribution from an exclusively radical process. The hexene-1 results are in qualitative accord with those previously reported for this olefin, while the differences noted here between this terminal olefin and the nonterminal olefins, hexene-2 and hexene-3, are qualitatively similar to the differences reported previously for butene-1 and butene-2.